A weak spot in a vessel is typically repaired by means of a welded repair, such as an overlay of welding material. The welding material is laid down over the weak spot in several layers until the wall has a sufficient thickness and strength. Laying down the welding material can be done manually, or by using an automatic welding machine which is mounted in the neighbourhood of the weak spot. Alternatively, the welded repair is effected by removing portions of the vessel wall in the vicinity of the weak spot and inserting metal panels. In some cases, metal panels may be fixed to the vessel wall over the weak spot.
When making a welded repair, the heat of the welding is transferred to the wall of the vessel, and thus local hot spots are formed in the wall. This causes local deformation of the wall, which can cause distortion of the vessel.
To minimize such distortion, the vessel can be supported with a crane, however, for a very tall vessel, such as a distillation tower, this would require an expensive tall crane. It may also be impractical or unsafe to use a crane because of the location of the vessel in a plant.
Support elements may be arranged over the weak spots, wherein the ends of of the support elements are fixed to the wall of the vessel. Such support elements are sometimes called "strong-backs." The support elements locally strengthen the wall of the vessel and support the vessel during the repair. The number of support elements is determined by the amount of support that is required.
An advantage of such support elements is that they are easily applied. However, a disadvantage is that they also restrict thermal extension of the vessel wall at the place where the welded repair is made which causes high thermal stresses in that part of the wall where the welded repair is made.